Optical fiber has found widespread application in longhaul signal transmission such as, for example, between cities. It has become apparent that optical fiber not only can advantageously be used as a long haul transmission medium but also economically in local networks including distribution, service or drop, and indoor cable. As a result, end-to-end optical communications is fast becoming a reality.
Optical fiber cables designed for long-haul applications typically are not well suited for use in the local distribution network. For instance, such cables typically are designed to have a medium to relatively high fiber count, are frequently relatively rigid and have a relatively large bending radius, and tend to be relatively costly to manufacture. On the other hand, cable for use in local distribution networks should have a low fiber count, e.g., less than about ten fibers, should be flexible, should be usable in a variety of environments, should be installable by existing procedures and equipment and should be easy to manufacture and low in cost.
A sought-after cable for such use should have desired performance properties. For example, it should have a relatively high tensile axial loading capability, have a relatively low minimum bend radius, have sufficient stiffness to minimize bend losses, and have an operating temperature range of about -40.degree. C. to +60.degree. C. The cable should be cushioned sufficiently to withstand repeated impacts by vehicles on structures routed across roadways during installation. Also, the sought after cable must be capable of being rendered water-resistant for buried use and not affected adversely by cable filling materials which provide such capability to the cable. Inasmuch as in some instances it will connect to customers' premises, the cable must be capable of being made flame retardant.
The prior art includes optical fiber cables that are designed for use in the local distribution network. For instance, S. Kukita et al, Review of the Electrical Communications Laboratories, Volume 32 (4), pp. 636-645 (1984) review the design and performance of optical drop and indoor cables, and disclose an indoor cable comprising a coated fiber surrounded by a polyvinyl chloride (PVC) jacket in which are embedded four steel wires. Such a design has some shortcomings, including a need for electrically grounding the steel wires, the difficulty in achieving good coupling between the jacket and the steel wires and the possibility of causing bending-induced stresses on the optical fiber.
A commercially available optical fiber cable that can find use in local distribution plant comprises a central steel or plastic strength member surrounded by a polyurethane jacket, a multiplicity of optical fiber-containing loose tubes stranded around the jacket, a polyethylene inner jacket surrounding the tubes, and a metallic armor and a polyethylene outer jacket surrounding the inner jacket. In such a structure, the fibers cannot be on the neutral axis of the cable when the cable is bent, requiring stranding of the loose tubes, which in turn complicates manufacture. Furthermore, in such a design, the strength member is not well coupled to the outer jacket.
Another commercially available optical fiber cable for use in a local distribution network includes a central coated steel wire strength member and multiplicity of plastic tubes arranged around the central strength member, each tube containing one buffered optical fiber as well as a moisture resistant filling compound. This core is surrounded by aramid yarn which is said to take up the greater part of any tensile load applied to the cable. The yarn layer in turn is surrounded by a polyethylene jacket. Such cables are useful as aerial or duct cable. For buried use, a polyurethane inner jacket replaces the polyethylene jacket, and a steel tape armoring and a polyethylene outer jacket surround the inner jacket. The tubes must be stranded inasmuch as the fibers are not on the neutral axis of the cable when the cable is bent. Furthermore, axial stresses are poorly coupled from the outer surface of the cable to the central strength member.
Inasmuch as present design buried service cables still have some drawbacks, efforts have continued to manufacture a buried service cable that provides an optical fiber transmission path onto customer premises. An optical fiber communications transmission cable of U.S. Pat. No. 4,723,831 which issued on Feb. 9, 1988 in the names of B. D. Johnson et al comprises a core comprising one or more optical fibers, a core wrap surrounding loosely the core, and three groups of longitudinally extending non-metallic strength members. Each group comprises one or more strength members completely embedded in a jacket and coupled thereto. The strength members are spaced equally in a circumferential direction. The jacket typically is not coupled to the core wrap to any substantial degree. For some applications of the cable, the voids between the optical fibers and the core wrap are filled with a waterblocking material. For indoor use, the cable advantageously includes flames retardant materials. The optical fibers typically have excess length, i.e., in any length l.sub.c of cable, the length l.sub.g of any optical fiber in the cable is greater than l.sub.c by some relatively small amount. Each one of the strength members embedded in the jacket includes a multiplicity of filaments impregnated with a material that is compatible with that of the jacket, resulting in substantial coupling between the jacket and the strength members embedded therein.
The cable of U.S. Pat. No. 4,723,831 also may comprise metallic armoring surrounding the jacket which becomes an inner jacket, surrounded by a polymer outer jacket. The outer jacket and the armoring are applied advantageously such that they are mechanically coupled to the inner jacket. This assures that longitudinal stresses that act on the outer surface of the outer jacket are transmitted to, and substantially borne by, the strength members.
Although the just-described cable overcomes some prior art problems, it is somewhat difficult to access the core without damaging the fibers. Also the strength members may wander in position as the inner jacket is extruded thereabout. Further, when the cable is subjected to bending, at least one of the strength members is put into compression and may cause unsightly bumps in the cable jacketing.
What still is needed is a buried service cable that extends from a distribution closure to a residence, that overcomes drawbacks of prior art offerings and that has suitable strength. Although placement of strength members in a jacket is preferred, care must be taken so that the extrusion of jacket material does not cause the strength members to be displaced.
Furthermore, and as mentioned hereinbefore, the sought-after cable should be capable of acceptable performance throughout a wide range of environmental conditions. At low temperatures, the optical fibers should be sufficiently decoupled from an enclosing sheath system to avoid undue buckling forces which otherwise could be applied to the fibers. Also, it becomes important for a craftsperson to be able to access easily the core and optical fibers therein. Further, after the fibers have been accessed, it should be relatively easy to remove any buffer coating from the glass fiber. This capability must exist with a very low probability that a craftsperson may damage the core which includes the optical fibers.
What is needed and what seemingly is not provided by the prior art is an optical fiber cable which may be used in the local area network to provide service to customer premises. The sought-after cable must have well defined strength member dispositions, must provide easy access to its fibers, must not sustain undue losses when subjected to temperature extremes, must be rugged in flexure, impact, compression and torsion and must be capable of being flame retardant and include waterblocking provisions.